This invention relates to improved X-ray tubes in general and more particularly to X-ray tubes with efficient rotation drives for the anode, with a rotational multiple focus cathode, and with a compact metal tube envelope.
The X-ray tube has become essential in medical diagnostics, medical therapy, and many parts of industry for material testing and material analysis. In X-ray medical diagnostics, rotating anode X-ray tubes are used almost exclusively to meet the demand for high quality X-ray imaging. On the other hand, many mobile X-ray units still utilize stationary X-ray tubes, but these units are very limited in application and, therefore, many mobile units now also incorporate a rotating anode tube.
Rotating anode X-ray tubes, which are capable of higher output for high quality imaging, were developed about 1920 as an improvement over stationary anode X-ray tubes with limited output. The first rotating anode X-ray tubes incorporated an induction motor operating from a standard 60 or 50 HZ power source, thus rotating the anode at about 3,300 rpm from a 60 cycle power. In the early 1950's, the so-called high speed (10,000 rpm) rotating anode X-ray tubes were developed to improve radiographic imaging further. In addition, in the 1950's a new area of radiology was developed, i.e. the study of the vascular system by injecting radio-opaque contrast media into the vascular system while simultaneously making single or multiple rapid sequence exposures. In addition, with the arrival of the image intensifier, X-ray motion picture studies were possible, putting even higher demands on the rotating anode X-ray tubes. X-ray tubes have become more complex and more expensive, and perhaps more fragile and prone to failure, in an effort to meet these demands.
Present rotating anode X-ray tubes have a cathode consisting of one or two filaments with corresponding focus cups, and a rotating anode assembly. These are mounted inside an all glass or a metal/glass evacuated tube envelope and the envelope is mounted inside an X-ray tube housing. The housing is filled with insulating oil, includes a heat expansion system and also incorporates the stator of an AC squirrel cage anode drive motor. The stator is generally concentric about the rotor of the anode drive motor, the rotor being part of the rotating anode assembly inside the vacuum tube envelope. Thus, the stator is spaced from the rotor by the thickness of the tube wall plus necessary clearances, which makes the squirrel cage motor inefficient and heat-producing.
X-rays are produced when the cathode filament is heated to a desired temperature and high voltage is applied between the cathode and anode. Maximum tube voltages of 100 KV, 125 KV or 150 KV across the cathode/anode gap are typical. Electrons flow in a narrow beam from the cathode to the anode at high acceleration and speed dictated by the high voltage. The electrons hit the anode and produce X-rays; however, only approximately 1% X-rays versus approximately 99% heat are produced for the amount of power applied. Due to this inefficienty in X-ray production, heat control and cooling are of major concern when designing modern high performance rotating anode X-ray tubes.
Most rotating anode X-ray tubes have two cathode filaments providing a smaller and a larger focus, depending upon which filament is heated. However, various X-ray techniques require differing foci, and typical nominal focal spot sizes required are: 0.1 mm.sup.2, 0.3 mm.sup.2, 0.6 mm.sup.2, 1.0 mm.sup.2, 1.2 mm.sup.2, 1.5 mm.sup.2, 1.8 mm.sup.2 and 2.0 mm.sup.2. Since present tubes provide only two foci, e.g. 0.6 mm.sup.2 and 1.2 mm.sup.2, one X-ray examination room for special procedure studies may require four X-ray tubes with each tube having different combinations of foci.
The most difficult design criteria of the high performance rotating anode X-ray tube is the anode/rotor structure. Today rotating anode X-ray tubes apply the AC squirrel cage induction motor principle, which basically is a two-pole frequency dependent motor. Therefore, 60 Hz provides a "standard" speed of approximately 3,300 rpm and 180 Hz provides a "high" speed of approximately 10,000 rpm. Since the rotor has to operate inside the vacuum, no conventional lubricants can be used for the ball bearings used to mount the anode/rotor structure. In addition, most manufacturers use the ball bearings as current carrier to the anode, and the current with many of the newer tubes may be in the range of 1,500 mA (milliamperes) to 2,000 mA at an anode voltage of for instance 100 KV. The current often pits the bearing surfaces, leading to vibration and failure. It should also be noted that, because of the two-speed motor drive, the anode is often rotated for fairly long periods at speeds higher than required by the operating power of the tube, and this also leads to tube failure.
Today, most all of the rotating anode X-ray tubes have the anode/rotor structure on high voltage anode potential, where the stator is at ground or near ground potential inside the tube housing surrounded by the insulating oil and other insulating material. A recent X-ray tube insulates the anode from the rotor. However, in either structure, there is a large gap between the stator and the rotor and a lot of stator power is required for fast acceleration and deceleration of the anode.
Radiography is a common medical X-ray procedure and may consist of: (a) radiography only, in which high or medium high power is directly applied to the X-ray tube after the anode is at standard or high speed, as required, and the filament is at the selected mA (filament temperature dictates the mA); or (b) combined radiography and fluoroscopy, in which television viewing precedes a radiographic exposure. Television viewing takes place at low tube power of around 100 Watts to 300 Watts but is continuous for long intervals followed by a high powered pulse of tube power for making a radiograph. This pulse, of from instance 100 Kilowatts, of course requires high speed rotation of the anode. The radiologist likes to instantly record what he may see on the television screen. Therefore, the time from fluoroscopy/television viewing to the radiographic pulse should be as short as possible. Less than one second changeover is desirable, but almost impossible with the new high performance high speed tubes. Even for a direct radiograph, such as a chest X-ray where no television viewing precedes the exposure, it is desirable to have a short time of less than one second start-up from zero to maximum anode speed, because the patient has to take a deep breath and hold it. In infant radiography, the technician may watch the infant's breathing and trigger the exposure when breathing of the infant is at a desired position. There are also automatic trigger devices, which allow selecting of the exposure trigger at any breath positon or for instance at any heart cycle position.
From the rotating anode point of view, these techniques require either a short start time, which requires a lot of power to the stator of the anode drive motor, or an advance start of long time with low power to the stator. The low power long advance start time system is shortening the tube life due to long rotation periods, and the other high power short start time systems create undesirable housing heat units which may be so high combined with the heat units coming from the anode that forced oil or water circulation may be required for keeping the X-ray tube housing temperature within its safety limits. In addition, the starter circuitry for fast acceleration of the rotor/anode structure is complex and expensive.
Overall, X-ray tubes have been developed to the point where they are highly useful but are also highly specialized, sophisticated and expensive structures with a relatively short use expectancy in view of their cost.